Add ES6 module build to NPM package

[nhunzaker]

Followup from a tweet from Unpkg (unpkg?) about ES6 module support:
https://twitter.com/unpkg/status/905239560988221440

For fun, try using import * as d3 from '//unpkg.com/d3?module inside a <script type=module> and watch those modules load!

It would be neat if you could just import { Component } from 'https://unpkg.com/react?module' in browsers like Safari (and soon Chrome). As far as I understand the problem, we'd just need to export an ES6 build target from Rollup.

I use "just" lightly. Hopefully it's that simple; it's probably more complicated. In any case, now we have an issue for it!

[aweary]

I believe this is a duplicate of #10021 :)

[nhunzaker]

Ah man. Look at me... Thanks!